Conventionally known are various machine tools for performing various machining operations, such as cutting, turning, boring, electrical discharge machining, etc. under the control of a numerical control device. In general, the numerically-controlled machine tools of this type has a function to automatically stop the operation of the machine tool and automatically display an alarm message indicative of the kind of trouble when the trouble occurs during the operation. Therefore, the kind of trouble can be quickly specified. The operation of the machine tool, however, involves transfer of various control information signals, including various commands delivered from the numerical control device, various feedback signals delivered from sensor systems of the machine tool, and various commands and data manually inputted through a control panel, so that it is difficult to grasp the progress of the operation before the occurrence of the trouble with speed and accuracy. Accordingly, the investigation of all the causes of troubles requires labor except those ones, such as machining program errors, which can be specified relatively easily. In order to restart the operation of the machine tool in the same operating conditions as when trouble is caused, moreover, the operating conditions at the time of occurrence of the trouble must be ascertained. However, if an operator operates a reset button by mistake, for example, the operating conditions will be initialized. It is difficult, therefore, to grasp the operating conditions at the time of occurrence of the trouble in accordance with the contents of a machining program only. This constitutes a hindrance to the restart of the operation.